•Full tensor properties of [111]C poled KNNT-2 crystal were firstly reported.•Orientation dependence of d33*, ε33*, s33* and k33* in (01¯1)C plane were calculated.•Intrinsic and extrinsic contributions to electric properties were discussed.Complete sets of elastic, piezoelectric and dielectric constants of [111]C poled (Na,K)(Nb, Ta)O3 single crystal have been measured. The results show that the crystal with “3O” domain structure exhibits high shear but low longitudinal piezoelectric properties (d15=140 pC/N, k15=0.569, d33=64 pC/N and k33=0.450), which is similar to the “1O” single domain state, but different from the “4O” multi-domain state. The orientation-dependent longitudinal properties, e.g.  , d33*, ε33*, s33* and k33* in the (01¯1)C plane have been calculated. The maximum piezoelectricity exists at the position of 38° away from [011]C, which is close to the [111]C direction. By comparing the measured and calculated parameters, it can be concluded that the piezoelectricity is dominated by the intrinsic contribution while the dielectricity is controlled by the extrinsic contributions.

•Optical properties for KNN-based single crystal was studied for the first time.•Temperature dependence of band gap energies and phonon energy was calculated.•Structure of KNNT single crystal was characterized by Raman spectroscopy.The optical properties of KNN-based single crystals were investigated firstly in this work. The optical transmittance of (K0.56Na0.44)(Nb0.77Ta0.23)O3 (KNNT) single crystal was measured at different temperatures. The band gap energies of direct transition Egd, indirect transition Egi, and the phonon energies Ep involved in the indirect transition were calculated based on the optical transmittance spectra. It was found that Egd and Egi show abrupt change at orthorhombic–tetragonal phase transition temperature (TO–T) at 119 °C due to the change of crystal lattice, while Ep varies rapidly at around 60 °C. In order to further understand the phase transition behavior and the assistant phonons in the indirect transition, the KNNT single crystal was characterized by using Raman spectroscopy. It is clarified that the υ1 vibration mode is involved in the indirect transition below 60 °C and υ5 mode is involved above 60 °C. The Raman scattering of KNNT crystal shows distinct behavior in different phases. The phase transitions at TO–T = 119°C and TC = 291 °C can be confirmed respectively as orthorhombic–tetragonal and tetragonal–cubic by the discontinuities of the Raman peaks’ intensities and positions.

A high quality orthorhombic (K, Na, Li)(Nb, Ta)O3 single crystal with a large size of 9 × 9 × 11 mm3 was successfully grown using the top-seeded solution growth method. Its large size allowed us to perform comprehensive investigation on this crystal. A complete set of elastic, dielectric and piezoelectric coefficients was determined using combined resonance and ultrasonic methods. The longitudinal piezoelectric coefficient d33 and the transverse piezoelectric coefficient d31 of this lead-free piezoelectric single crystal are as high as 354 pC N−1 and −163 pC N−1, respectively, which are two times higher than those of KNN-TL ceramics and are comparable with those of PZT5. The electromechanical coupling factor k33 is as high as 82%, much higher than those of KNN-TL ceramics and PZT5. The coefficient d33,strain was found to be as high as 672 pm v−1 and the extrinsic contribution (~22%) was investigated by Rayleigh analysis. These high piezoelectric properties make this single crystal a promising candidate for replacing lead-based piezoelectric materials.

In order to clarify the influence of excess ions in A or B sites on perovskite (K,Na)NbO3 ceramics, various compositions of (K0.48Na0.52)Nb1+x%O3 (ABO3) ceramics where x is in the range of ±1 % were prepared by conventional solid state method and their densification, structure, dielectric and piezoelectric properties were investigated. Results showed that a small amount of excess A-site ions could compensate for the deficiency of K and Na ions in A-sites caused by volatilization resulting in good piezoelectric properties. The ceramics with x = −0.1 exhibited optimum piezoelectric properties with d33 = 127pC/N and kp = 0.41. However, presence of too much alkali elements (x < −0.5) led to deterioration of density, dielectric and piezoelectric properties, although the crystal structure was not changed. The electrical properties, on the other hand, were not sensitive to the B-site excess ions. These results are expected to be very useful for further designing of (K,Na)NbO3-based ceramics as lead-free alternatives to piezoelectric materials.

•High quality large size Li, Sb, Ta co-doped (K,Na)NbO3 single crystal was successfully grown.•The tetragonal crystal shows homogenous composition.•The [001]C oriented single crystal shows relatively good piezoelectric properties.•The electromechanical coupling factor kt of the crystal exhibited very good thermal stability.In this work, a large size lead-free piezoelectric single crystal, (K,Na,Li)(Nb,Ta,Sb)O3 (KNLNTS) with the dimensions of 8.5×8.5×13.5 mm3 was successfully grown by the top-seeded solution growth method. This KNLNTS single crystal with high compositional homogeneity is in the tetragonal phase at room temperature. The Curie temperature TC of the tetragonal-cubic phase transition temperature is 210 °C. The piezoelectric coefficients and electromechanical coupling factors of the [001]C oriented KNLNTS single crystal are d33=172.55 pC/N, d31=−71.90 pC/N, k31=0.327, k33=0.523, and kt=0.541. In addition, the crystal shows good thermal stability so that it can be used for making high temperature electromechanical devices.